In the prior art, switching power supplies characterized by small sizes, light weights, and high power efficiencies have been widely used as power supplies of electronic equipment and so on. As this kind of switching power supply, the following will describe a resonant switching power supply which obtains a direct current output by switching a direct current input into a constant voltage and has a serial-parallel resonance circuit in a switching control section to satisfactorily obtain a constant voltage over a wide range of changes of a load capacity.
As a switching power supply capable of changing a direct current output voltage Vo over a wide range, supplying a stable direct current output voltage to a load, and minimizing a power loss, a resonant switching power supply is available which has a serial-parallel resonance circuit Z2 as shown in FIG. 6 (for example, see International Patent Publication WO 2005/109618 A1).
The following will describe the resonant switching power supply of FIG. 6.
A resonant coil L1 is connected in series with a resonant capacitor C2 and a resonant capacitor C3 is connected in parallel with the primary winding of a switching transformer T1. The values of the resonant capacitor C2 and the resonant coil L1 which are connected in series are mainly set for a heavy load, that is, a low frequency used when a direct current output has a large load current, and the value of the resonant capacitor C3 connected in parallel is set for a light load, that is, a high frequency used when the direct current output has a small load current.
In other words, as a load capacity, a plurality of peaks of frequency characteristics, that is, resonance characteristics having a plurality of peaks are obtained for a wide range of changes from when an output voltage is high and the load is heavy (a large load current), that is, from when the direct current output to the load has a large load current to when the load is minimized, that is, the output voltage is low and the direct current output has a small load current. Thus the overall resonance characteristics cover a wide band and switching frequencies are more stably changed than in the case of single-peak resonance characteristics, so that the direct current output can be stably outputted.
In recent years, such a resonant switching power supply has been requested to respond to a large capacity load. Circuit systems corresponding to large capacity loads are divided broadly into a system in which the circuit system of the resonant switching power supply of FIG. 6 is used and the circuit configuration is simply increased in capacity to achieve higher power (for example, see Japanese Patent Laid-Open No. 2006-042545) and a system in which the circuit of the resonant switching power supply of FIG. 6 is applied as a single power supply circuit U1 as shown in FIG. 7 and the plurality of power supply circuits U1 are connected in parallel to increase power (for example, see Japanese Patent Laid-Open No. 7-184322).
Further, in the resonant switching power supply of FIG. 7, the plurality of power supply circuits U1 corresponding to a capacity have power supply input sections connected in parallel and power supply output sections connected in parallel, thereby increasing a load current.
Moreover, for the control of the power supply circuits U1, various control systems are available which include a system in which a control circuit S2 is provided to control control circuits S1 of the power supply circuits U1 as shown in FIG. 7 and a system in which the control circuits S1 of the power supply circuits are mutually controlled by a mutual control function (for example, see Japanese Patent Laid-Open No. 7-184322).